Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Recently, efforts have been devoted to identifying ways to operate electrochemical fuel cells using other than pure hydrogen as the fuel. Fuel cell systems operating on pure hydrogen are generally disadvantageous because of the expense of producing and storing pure hydrogen gas. In addition, the use of liquid fuels is preferable to pure, bottled hydrogen in some mobile and vehicular applications of electrochemical fuel cells.
Recent efforts have focused on the use of impure hydrogen obtained from the chemical conversion of hydrocarbon fuels to hydrogen. However, to be useful for fuel cells and other similar hydrogen-based chemical applications, hydrocarbon fuels must be efficiently converted to relatively pure hydrogen with a minimal amount of undesirable chemical by-products, such as carbon monoxide.
Conversion of hydrocarbons to hydrogen is generally accomplished through the steam reformation of a hydrocarbon such as methanol in a reactor commonly referred to a catalytic hydrocarbon reformer. The steam reformation of methanol is represented by the following chemical equation: EQU CH.sub.3 OH+H.sub.2 O+heat.revreaction.3H.sub.2 +CO.sub.2 ( 1)
Due to competing reactions, the initial gaseous mixture produced by steam reformation of methanol typically contains from about 0.5% to about 20% by volume of carbon monoxide and about 65% to about 75% hydrogen, along with about 10% to about 25% carbon dioxide on a dry basis (in addition, water vapor can be present in the gas stream). The initial gas mixture produced by the steam reformer can be further processed by a shift reactor (sometimes called a shift converter) to reduce the carbon monoxide content to about 0.2% to about 2%. The catalyzed reaction occurring in the shift converter is represented by the following chemical equation: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 2)
Even after a combination of steam reformer/shift converter processing, the product gas mixture will have minor amounts of carbon monoxide and various hydrocarbon species, each present at about 1% or less of the total product mixture. A variety of conventional treatment processes may be employed to remove many of the hydrocarbon and acid gas impurities generated during the steam reformer/shift converter process. However, such conventional treatment methods are generally incapable of reducing the carbon monoxide content of the gases much below 0.2%.
In low temperature, hydrogen-based fuel cell applications, the presence of carbon monoxide in the inlet hydrogen stream, even at the 0.1% to 1% level, is generally unacceptable. In solid polymer electrolyte fuel cells, the electrochemical reaction is typically catalyzed by an active catalytic material comprising a nobel metal such as platinum. Carbon monoxide absorbs preferentially to the surface of platinum, effectively poisoning the catalyst and significantly reducing the efficiency of the desired electrochemical reaction. Thus, the amount of carbon monoxide in the hydrogen-containing gas mixture produced by a steam reformer/shift converter process for use in electrochemical fuel cells should be minimized, preferably to an amount significantly lower than the approximately 1% achieved using conventional steam reformation methods. The present selective oxidizing method and apparatus reduce the amount of carbon monoxide in a hydrogen-containing gas stream to a level suitable for use in electrochemical fuel cells, generally significantly less than 100 ppm.
In the present selective oxidizing method and apparatus, it is believed that at least three competing reactions occur, which are represented by the following chemical equations:
1. The desired oxidation of carbon monoxide to carbon dioxide: EQU CO+1/20.sub.2 .revreaction.CO.sub.2 ( 3) PA1 2. The undesired oxidation of hydrogen to water: EQU H.sub.2 +1/2.sub.2 .revreaction.H.sub.2 O (4) PA1 3. The undesired reverse water shift reaction: EQU CO.sub.2 +H.sub.2 .revreaction.H.sub.2 O+CO (5) PA1 (a) a primary reaction chamber having disposed therein an amount of catalyst for promoting oxidation of carbon monoxide to carbon dioxide and further comprising a first inlet and a second inlet; PA1 (b) a second reaction chamber having disposed therein an amount of catalyst for promoting oxidation of carbon monoxide to carbon dioxide and further comprising a first outlet, the second reaction chamber in fluid communication with the primary reaction chamber, and the first outlet in fluid communication with the first inlet through at least a portion of each of the first and second reaction chambers; PA1 (c) a third reaction chamber having disposed therein an amount of catalyst for promoting oxidation of carbon monoxide to carbon dioxide and further comprising a second outlet, the third reaction chamber in fluid communication with the first reaction chamber, and the second outlet in fluid communication with the second inlet through at least a portion of each of the first and third reaction chambers; PA1 (d) a first valve means for rendering the second inlet and the second outlet substantially impassable to fluid flow when the first inlet and the first outlet are passable to fluid flow; and PA1 (e) a second valve means for rendering the first inlet and the first outlet substantially impassable to fluid flow when the second inlet and the second outlet are passable to fluid flow. PA1 (a) a primary reaction chamber having disposed therein an amount of catalyst for promoting oxidation of carbon monoxide to carbon dioxide and further comprising a first inlet, a second inlet, a first outlet, and a second outlet, wherein the first inlet is in fluid communication with the first outlet and the second inlet is in fluid communication with the second outlet; PA1 (b) a secondary reaction chamber having disposed therein an amount of catalyst for promoting carbon monoxide to carbon dioxide and further comprising a secondary inlet and a secondary outlet, wherein the secondary inlet is in fluid communication with each of the first outlet, the second outlet, and the secondary outlet through at least a portion of each of said primary and secondary reaction chambers; PA1 (c) a first valve means for rendering the second inlet and the second outlet substantially impassable to fluid flow when the first inlet and the first outlet are passable to fluid flow; PA1 (d) a second valve means for rendering the first inlet and the first outlet substantially impassable to fluid flow when the second inlet and the second outlet are passable to fluid flow. PA1 (a) a primary reaction chamber having disposed therein an amount of catalyst for promoting oxidation of carbon monoxide to carbon dioxide and further comprising a first primary inlet, a primary second inlet, a first primary outlet, and a second primary outlet, wherein the first primary inlet is in fluid communication with the first primary outlet and the second primary inlet is in fluid communication with the second primary outlet; PA1 (b) a first secondary reaction chamber having disposed therein an amount of catalyst for promoting carbon monoxide to carbon dioxide and further comprising a first secondary inlet and a first secondary outlet, wherein the first secondary inlet is in fluid communication with each of the first primary outlet and the first secondary outlet through at least a portion of each of said primary and first secondary reaction chambers; PA1 (c) a second secondary reaction chamber having disposed therein an amount of catalyst for promoting carbon monoxide to carbon dioxide and further comprising a second secondary inlet and a second secondary outlet, wherein the second secondary inlet is in fluid communication with each of the second primary outlet and the second secondary outlet through at least a portion of each of said primary and second secondary reaction chambers; PA1 (d) a first valve means for rendering the second primary inlet and the second primary outlet substantially impassable to fluid flow when the first primary inlet and the first primary outlet are passable to fluid flow; PA1 (e) a second valve means for rendering the first primary inlet and the first primary outlet substantially impassable to fluid flow when the second primary inlet and the second primary outlet are passable to fluid flow. PA1 (a) measuring the flow rate of the fuel gas stream introduced to the reaction chamber at the fuel stream inlet; PA1 (b) actuating the oxygen-containing gas stream inlet valve such that an amount of oxygen-containing gas is introduced to the reaction chamber at a flow rate in direct proportion to the flow rate of the fuel gas stream as measured in step (a); PA1 (c) actuating the oxygen-containing gas stream inlet valve to increase the flow rate of the oxygen-containing gas stream into the reaction chamber from the flow rate established in step (b) when the difference between the temperature measured by the first thermocouple and the temperature measure by the second thermocouple is greater than a pre-set control temperature difference; and PA1 (d) actuating the oxygen-containing gas stream inlet valve to decrease the flow rate of the oxygen-containing gas stream into the reaction chamber from the flow rate established in step (b) when the difference between the temperature measured by the first thermocouple and the temperature measure by the second thermocouple is less than a pre-set control temperature difference.
One of the most commonly used selective oxidizer designs uses an adiabatic catalyst bed to react the carbon monoxide with oxygen supplied by an oxygen-containing gas (e.g,, air). Catalyst loading, bed space velocity, and air flow are selected to control the temperatures in the bed so that bed size is minimized while the selectivity of the reaction to consume carbon monoxide is maximized.
Performance of the selective oxidizer catalyst gradually decays due to the gradual blanketing of the catalyst active sites with carbon monoxide. After a period of time, this decrease in catalyst performance caused by carbon monoxide results in a rapid increase in the carbon monoxide concentration of the selective oxidizer exit gas stream which is fed as the inlet stream to the fuel cell assembly. In conventional selective oxidation methods, blanketing of the selective oxidizer catalyst by carbon monoxide can be compensated for by increasing the catalyst bed temperature. However, while an increase in the bed temperature helps to compensate for the loss of catalyst activity, it also results in the loss of reaction selectivity, and thus increased hydrogen consumption which is highly undesirable in fuel cell applications.